Most simply, force equals mass times acceleration. Although you are given the initial momentum, rather than the initial speed, since momentum equal mass times speed, and mass stays contant in this problem, just divide the "change in momentum" (from 3.0 x 104 to 0) by the time required.

The second law is that the acceleration of an object is dependent on two factors - the net force acting upon the object and the mass of the object...
But in relation to the question there are a few variables missing..

Look at it like this...Starting with

[tex]F = m a[/tex] where [tex]a = \frac{d}{dt} v[/tex]

[tex]F = m \frac{d}{dt} v[/tex] since mass is constant, we can pull it inside the derivative